1. Field of the Invention
The present invention relates to the field of establishing computer networks using cable modems. Specifically, the present invention allows a client communicating with a server using separate upstream and downstream devices to designate one or more devices as the devices to receive data from the server.
2. Description of Related Art
Currently, most home personal computers (clients) are connecting with the Internet and other on-line services using the public telephone network. Most often, data is transferred using Transmission Control Protocol/Internet Protocol (TCP/IP) implemented over such protocols as the Point-to-Point Protocol (PPP) or Serial Line IP (SLIP). PPP and SLIP allow clients to become part of a TCP/IP network (such as the internet) using the public telephone network and either an analog modem or an Integrated Services Digital Network (ISDN) device. Clients connect to a network by "dialing-up" a Point of Presence (POP) server, which then assigns the client an IP address.
The public telephone network has a switched point-to-point architecture and only offers relatively low bandwidth as it was originally designed for analog voice communication. Thus, it does not scale well to the delivery of broadband data such as multimedia. As a result, there are several efforts to create a broadband data delivery infrastructure for client applications. Such an infrastructure, when combined with the increasingly powerful clients that are now available, will enable the delivery of rich multimedia programming to the home.
Broadband data delivery may be accomplished over a variety of different delivery infrastructures. Of these, perhaps the most promising is the infrastructure currently used to deliver cable television. Recent advancements in radio frequency modulation and demodulation technology, along with a large base of cable television subscribers, has made cable television service providers a strong candidate for becoming the preferred provider of broadband services to the home. In the broadband network architecture, a client will be continuously connected to the broadband network and will be in communication with one or more servers at all times.
However, as the cable television network was originally intended only for transmitting "downstream" data from a "headend" server located at the cable television service provider's site to one or more subscribers/users (i.e., the network was designed for a one-to-many transmission of information), no provision was made for receiving data from the equipment (i.e., clients) located at the users' locations.
A solution has been proposed to achieve two-way communication of data using the existing cable television infrastructure. "Downstream data", defined to be data sent from a "headend server" to a client, is transferred over coaxial cable from the headend server into the home and to the user's client PC, while "upstream data", defined to be data sent from the client to the headend server, is transferred over the public telephone network. The asymmetrical allocation of upstream/downstream bandwidth is acceptable for most applications as the majority of users requires a larger downstream bandwidth compared to the upstream bandwidth (i.e., most users are "data consumers" rather than "data generators").
In operation, downstream data is received by a client using a "one-way" cable modem while upstream data is transmitted by an analog modem or an ISDN device, over the public telephone network, to the headend server via a Plain Old Telephone Service (POTS) server at the headend office. The POTS server forwards any upstream data sent by the client to the headend server for appropriate action (e.g., providing domain name server (DNS) services, simple mail transfer protocol (SMTP) services, gateway or proxy services, etc.).
The client, the POTS server, and the headend server communicate using TCP/IP. Data is transmitted in packets, where packets are defined as a block of data with appropriate transmission data attached in the form of a header and footer to be sent or received over a network. Downstream and upstream data are sent using the ethernet standard, as defined by the Institute of Electrical and Electronics Engineers (IEEE) 802.3, modulated for transmission over: (1) coaxial cable using the cable modem; or, (2) a telephone line using the analog modem or the ISDN device and PPP or SLIP protocols.
The cable modem and the analog modem/ISDN device will each have both an IP address and a media access control (MAC), "ethernet", address as the application layer uses IP addresses and the physical layer uses MAC addresses. MAC addresses are statically assigned to each device during manufacture, while IP addresses are dynamically assigned. To correlate MAC addresses to IP addresses, a look-up table conforming to the Address Resolution Protocol (ARP) is used by each machine to resolve the MAC address corresponding to an IP address.
An ARP table is updated periodically in response to conditions such as expiration/time-out of unused entries (i.e., a non-responsive machine) or the addition of a new machine. In the second situation, the ARP table is updated to contain a new entry to reflect the IP and MAC addresses of the new machine. For example, the headend server maintains its own ARP table for machines with which it communicates. When the headend server receives a packet from an "unknown" machine, the headend server will create a new ARP entry for that machine.
The problem with using separate devices for upstream and downstream data transfers is that the upstream device and the downstream device will both have a different IP address as there is the assignment of an IP address to the upstream device (i.e., the analog modem or ISDN device) in addition to the IP address already assigned to the cable modem. The problem involves the headend server, which, in response to data requests generated by the upstream device, will transfer downstream data to the upstream device. This is because the headend server will look-up the MAC address corresponding to the IP address of the requesting device in the ARP table and, finding that the MAC address belongs to the upstream device, the headend server will send the requested data to the upstream device as it is the IP address from which the request came.
For example, where the upstream device is an analog modem (dialing into a POTS server), and the downstream device is a cable modem, the client PC will have two IP addresses--one having been statically assigned to the cable modem, the other being assigned to the analog modem. Immediately after the client initiates a connection with the headend server via the POTS server, the client will be sent a "MAC-request" packet requesting IP and MAC address information. In response, the client will send a "MAC-reply" packet which will contain the client's IP and MAC address information. The POTS server will forward this information to the headend server, which will update its ARP table. Thereafter, when the client requests data from the headend server, the headend server will respond to the request by looking up the MAC address to which to send the requested data. However, as the lookup will be done by using the IP address of the ANALOG modem (i.e., the client PC's upstream device), the MAC address returned will be the MAC address of the analog modem. The headend server will thus select the analog modem instead of the cable modem as the receiving device, thereby undesirably transmitting downstream data to the analog modem, which is the slower connection.
Therefore, a solution needs to be provided for the above-identified situation where a client PC has two IP addresses for separate upstream and downstream devices and the transmission upstream and downstream data needs to be strictly segregated to each device.